Communication devices such as wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or Mobile Stations (MS). Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication network, sometimes also referred to as a cellular radio system, cellular system, or cellular network. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The wireless communications network covers a geographical area which is divided into cell areas, wherein each cell area may be served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
Extended Coverage
Cellular systems continuously improve the network performance by introducing new features and functionality. In GP-140421, “New SI on Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things”, GERAN#62, Vodafone, a new Study Item within 3rd Generation Partnership Project (3GPP) was started that aims, amongst other things, to improve the DL and UL radio coverage of General Packet Radio Service (GPRS)/Enhanced GPRS (EGPRS) by up to 20 dB. A way to enhance coverage may be to use blind transmissions of the same radio block with both the transmitter and receiver, being aware of how many repetitions may be used and how those repetitions may be transmitted in the overall frame structure. A radio block, which may be referred to herein also as a “block”, may be understood as a well confined structure for data and control message transfer that may be distributed over one or more physical resources, referred to as “bursts”. Herein, a “block” may also refer to a structure for transfer of synchronization signals and information. A “burst” may be understood as a well-defined physical resource onto which the fields of the block are mapped. Blind transmissions may be understood as a predetermined number of transmissions to support extended coverage. The transmissions may be sent blindly, that is, without feedback from the receiving end. To maximize the processing gain at the receiver, phase coherency at the transmitter, between repetitions, may be required.
Current Block Format
A block may be comprised of bits. A bit may be understood as the smallest unit of information in a digital information system. A bit is most commonly represented as either a 0 or a 1. The bits comprising the block may comprise information of different types. The types of information may comprise: training sequence, stealing flags, data and header and USF. The types of information may be organized in a particular manner. The manner in which the different types of information are organized is known as the format of the block, or block format. The types of information may be understood to be organized in fields. A field may be understood as a group of bits in a message carrying a type of information. A field may be comprised of a contiguous or non-contiguous group, or groups of bits when mapped onto the physical resource, that is, the burst(s).
The block format used for PACCH and PDTCH today in GSM may be divided into Stealing Flags (SFs), Uplink State Flags (USFs) and remainder of the block. The remainder of the block may be different depending on whether the block is a PACCH block or a PDTCH block, but may consist typically of a header and a data part, e.g., RLC/MAC header and RLC or control data, and its bit specific content differs from burst to burst. Since a radio block may be divided into 4 bursts the overall interleaving depth, which may be understood herein as the range over which an information field is distributed, of the data and header part is four bursts.
A USF may be understood as an identifier in an UL scheduling mechanism. The identifier may schedule a certain MS in a next UL radio block period. Among all MSs monitoring a DL radio block, only the single MS assigned the USF signaled in the DL radio block may be allowed to transmit in the next UL radio block period. For GMSK modulation, which is the modulation scheme used by GPRS devices, the USF bits may be mapped onto different bit positions in the four different bursts of a block, as shown in Table 1.
TABLE 1GMSK USF bit mappingBurstUSF bit position00, 51, 1021100, 35, 862 84, 19, 70368, 3, 52
A Stealing Flag may be understood as a signal for the type of radio block transmitted. The SF bits may be mapped onto the same bit positions in each burst, namely the two bit positions on either side of the training sequence. A training sequence may be understood as predefined sequence know by both transmitter and received, whose purpose may be understood as e.g. to facilitate estimation of the radio channel over which a burst may be transmitted.
The different fields of the current block format, also referred to herein as the legacy block format, are shown in FIG. 1, which is a schematic illustration of the current or existing block format. In the Figure, each burst is represented by a different row of bits. The top row 200 represents the burst number 0, the second row 201 represents the burst number 1, the third row 202 represents the burst number 2, and the fourth row 203 represents the burst number 3. A bit is represented in the Figure by a vertical rectangle. An individual bit 210 is marked. The type of information carried by each bit is illustrated with different patterns, as shown in the legend of the Figure. 58 bits are on each side of the training sequence bits, which are flanked by the SF bits. The USF bits are located in the bit positions listed in Table 1. The remaining bits correspond to data and header type of information. All the bits in any single burst of the bursts 200, 201, 202, 203 comprising the Training Sequence are referred to herein as the Training Sequence field 220. All the bits in the block comprising the SF are referred to herein as the SF field 230. All the bits in the block comprising data are referred to herein as the data field. All the bits in the block comprising the header are referred to herein as the header field. The data and header fields may be referred to herein together as the data and header fields 240, as shown in FIG. 1. All the bits in the block comprising the USF are referred to herein as the USF field 250.
Radio transmissions may be exposed to various impairments. One such impairment is the so-called frequency offset. A frequency offset may be understood as an offset between the frequency used by the transmitter and the receiver. A receiving device of a radio transmission may try to compensate for such frequency offset by detecting the offset and compensate for the same.
Blind transmissions of the same radio block have been suggested as a way to enhance radio coverage in existing systems because these if coherently combined in, they may improve the signal to noise ratio, e.g., with up to 3 dB per doubling of repetitions, and thereby increase the likelihood of correctly decoding a message. However, if such a frequency offset is not correctly estimated by the receiving device with existing methods, this may destroy the coherency and degrade the receiver processing gain when combining the repetitions. The processing gain in this context may be understood as the coverage performance improvement achieved by receiver algorithms. As a consequence the receiving device may not be reached in the extended coverage scenario, as an extension of the coverage may not be achieved.
When using multiple blind transmissions, a.k.a., blind physical layer transmissions or just blind transmissions, the receiver, such as a receiving device, may typically combine and accumulate several of these transmissions before calling the demodulator, and hence before it attempts to demodulate and decode the block. In this accumulation of multiple transmissions, there may be a need to do the accumulation in a particular way, so called coherently, in order not maximize the processing gain from these transmissions. In this process, a too high frequency offset in the reception may be detrimental to the overall performance. This is because a frequency offset leads to a phase drift over time which negatively impacts the possibility to combine the samples from repeated bursts in order to achieve a desired processing gain. Hence, there may be, typically, an attempt from the receiver to compensate for any frequency offset between transmissions that may result in a phase shift over time in the baseband representation of the signal.
To address this, an excessive number of repetitions may be needed, which results in a poor utilization of available radio resources. Furthermore, with an improper estimation of the frequency offset in the reception, the same frequency offset may apply when the receiver is transmitting in the opposite direction. Hence, an improper estimation in one direction may impact performance in both UL and DL. Therefore, existing methods for extended coverage result in poor performance of the wireless communications network.
Backwards Compatibility
Improper estimation of the frequency offset is not the only problem associated with the introduction of devices supporting extended coverage in a network. When introducing new features into a network, it may often be necessary to follow the requirement of backwards compatibility, i.e., that the previous network operation may not be impacted negatively by the introduction of the new feature.
This is because while the set of radio resources in the network may stay the same, devices of e.g., different capabilities depending on whether they support or not the new feature, may need to be allocated or scheduled on a common set of radio resources. That is, they may need to be multiplexed, or scheduled at different time instances, on the same time slot, or set of time slots.
In the particular case of Global System for Mobile Telephony (GSM)/General Packet Radio Service (GPRS) networks, for example, when introducing Enhanced General Packet Radio Service (EGPRS), providing as little impact as possible on the GPRS traffic was an important factor to take into account. One specific aspect that needed attention was the possible multiplexing of legacy GPRS devices and EGPRS devices onto the same physical resources, and that monitoring by legacy devices of the DL channel to see if they are scheduled in the UL, by the reading of the Uplink State Flag (USF) flag, was impacted to the least extent possible. As stated earlier, the USF signalled in the DL radio block may identify the single MS assigned to it that may be allowed to transmit in the next UL radio block period.
During a TBF, a connection established between a MS and a BS to enable packet exchanges between them in GPRS networks, the USF may be carried by two different channels, the Packet Data Traffic Channel (PDTCH), which may carry user data, and the Packet Associated Control Channel (PACCH), which may carry control signalling that may be needed to support the user data flow.
The problem of backwards compatibility is not new to GSM/EDGE. When introducing EGPRS, only partial multiplexing between GPRS and EGPRS devices was achieved. This means that both GPRS and EGPRS devices may be assigned the same resources in the network. However, both DL and UL scheduling of GPRS devices using 8-ary Phase Shift Keying (8PSK) modulation, the new modulation scheme introduced with EGPRS, is not possible, because the GPRS devices may only support Gaussian Minimum Shift Keying (GMSK) modulation. Still, the block format for EGPRS when using GMSK modulation was done to ensure that GPRS mobiles could read it.
This was specifically achieved by the BTS coding the Stealing Flags (SF) for PDTCH indicating CS-4 from GPRS. A GPRS device may therefore be able to interpret the SF as well as read the USF transmitted of EGPRS blocks transmitted with GMSK modulation. This is reflected in 3GPP TS 45.003 v12.0.0, “Channel coding”, for the coding description of MCS-1, which also applies to MCS-2, -3 and -4, where it may be noted that:
“Note: For a standard GPRS MS, bits q(0), . . . , q(7) indicates that the USF is coded as for CS-4.”
q(0), . . . , q(7) is here referring to the Stealing Flag bits.
According to the foregoing, lack of backwards compatibility with existing networks when introducing the extended coverage feature into a network may negatively impact the performance of the network due to unnecessary restrictions being imposed on to the network resource allocation and scheduling method, as e.g., multiplexing of devices supporting and not supporting extended coverage may not be possible.
Moreover, the frequency error offset associated with the blind repetitions used to extend the coverage in a network may result a failure to reach the devices that are aimed to be reached, hence degrading the performance of the network.